1. Field of the Invention
The field of art to which this invention pertains is crystalline aluminosilicate adsorbent production. More specifically, this invention relates to a solid zeolitic adsorbent having characteristics desirable for the separation of para-xylene from a mixture of C.sub.8 aromatic hydrocarbons.
2. Description of the Prior Art
There are numerous methods for the manufacture and ionexchange of various crystalline aluminosilicates, particularly the type X and type Y crystalline aluminosilicates, to yield products useful for effecting given hydrocarbon reactions or separations. In the method of this invention, a manufacturing method has been discovered whereby an adsorbent material is produced having superior properties with respect to the separation of para-xylene from a mixture of C.sub.8 aromatic hydrocarbons.
U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; and 3,663,638 show that adsorbents comprising crystalline aluminosilicates and containing barium and potassium are useful for separating para-xylene from a mixture of C.sub.8 aromatic hydrocarbons.
The treating of crystalline aluminosilicates with a caustic solution to achieve certain desired results has been recognized in the prior art. U.S. Pat. No. 3,326,797, for example, discloses a process for aqueous caustic treating of high silica zeolites having silica over alumina ratios between about 6 and 12, at treating conditions, for the sole purpose of removing a certain percentage of structural silica from the zeolite. The caustic treatment, at conditions to preferably retain a final SiO.sub.2 /Al.sub.2 O.sub.3 ratio greater than about 5.5, is found to increase the adsorptive capacity of the zeolite and to increase its catalytic activity. The caustic treating process of that reference patent is concerned only with etching or leaching of silica from the zeolite structure to achieve these characteristics and neither discloses nor suggests the addition of alkali metal cations to the zeolite structure during the treating process for any reason whatever.
I have discovered that ion-exchanging precursor particles comprising a type X or type Y zeolite and amorphous material as a binder with an aqueous solution of sodium hydroxide prior to the ion-exchange with potassium and barium or barium alone produces an adsorbent possessing faster adsorption-desorption rates for para-xylene when used to separate para-xylene from a mixture of C.sub.8 aromatic hydrocarbons. The reason for this is not entirely understood but it is hypothesized that the ion-exchange with aqueous sodium hydroxide replaces non-sodium cations, such as H+ or Group II-A cations, occupying exchangeable sites within the zeolite thereby permitting higher amounts of barium and potassium or barium alone to be added during a subsequent ion-exchange step.